Interaction of nitric oxide with the (1 0 0) surface of cobalt spinel nanocubes – A comprehensive DFT, atomistic thermodynamic, IR and TPD account

Abstract

Periodic spin-unrestricted DFT + U/PW91 calculations, atomistic thermodynamic modeling and IR and TPD measurements were used to study structure and stability of various NO adspecies on the (1 0 0) surface of Co3O4 nanocubes. A large variety of monodentate (η1), bridging bidentate (µ-η2) and bridging monodentate (µ-η1:η1) adducts were identified. The most stable were mononitrosyl (η1-ON-CoT) and dinitrosyl (η1-(ON)2-CoT) complexes of tetrahedral CoT centers, whereas the NO adducts of the octahedral CoO sites (η1-ON-CoO) were less favorable. The DOS structure, atomic partial charge and magnetization analysis revealed that their electronic structures can be epitomized as η1-ON-CoT↑↑ and η1-ON↑CoO↓↑. Thermodynamic ΘNO = f(pNO,T) diagrams were constructed and used for interpretation of a three-step NO desorption corresponding to decay of η1-ON-CoO at 60 ÷ 80 °C, decomposition of η1-(ON)2-CoT at 210 °C, and decay of η1-ON-CoT at 380 °C. The stability and spectroscopic signatures of these adspecies were ascertained in IR and NO-TPD measurements. The 1940 cm−1 band corresponds to chemisorption of NO on CoO sites, whereas for the CoT centers the mononitrosyl species appear at 1850 cm−1 and the dinitrosyls at 1895 and 1813 cm−1. The TPD profiles confirm the three NO desorption peaks in the temperature windows, predicted by the DFT and thermodynamic modelling.